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Dynamic thermal simulation in building ventilation design

The July 2018 issue of the CIBSE journal features a CPD module sponsored by Monodraught and written by Professor Tim Dwyer. It considersthe application of evolving dynamic thermal simulation software tools and their use in the assessment of building ventilation solutions, in particular using the CIBSE weather files to plan for all conditions over an extended period of time.

Thermal Dynamic Simulation

The development of Thermal Dynamic Simulation software has paved the way for building designers to carry out ever more detailed modelling simulations and ultimately, begin to quickly and accurately replicate the predicted performance of the building, long before it has been built.

Annual Weather Files

By carrying out a full thermal dynamic simulation, a user is able to model a whole year and specifically against data from an actual site specific weather file. The CIBSE weather files, which are available for 14 locations around the UK, use weather data obtained by the UK Meteorological Office from its different weather stations. For each weather file, these combine real life data measured over a 30 year baseline period, between 1984 – 2013, to gain an accurate representation for each location. The data includes dry & wet bulb temperatures (°C), atmospheric pressure (hPa); global & diffuse solar irradiations (W·h/m2), cloud cover (oktas), wind speed (knots) and wind direction (degrees).

By using the most representative CIBSE weather file for UK projects, this can allow a user to design their building in accordance with site specific data and the varying weather conditions that will occur throughout the year. Additionally to current weather file data set, CIBSE, in collaboration with UK Climate Impacts Programme (UKCIP), Arup and Exeter University, have created a series of predicted future weather files for 2020, 2050 & 2080. These have been based on the latest climate change projections that can allow designers to look at ‘future proofing’ their building, by carrying out simulations with the predicted Future weather scenarios.

Many of the different building design standards have also undergone major overhauls and updates, including to the overheating criteria. The Adaptive Thermal Comfort criteria acknowledges that, opposed to a single temperature being deemed as a set point for overheating for the entire non-heating season, this temperature limit should vary based upon the external air temperature.

Design Complexity

A key advantage with thermal dynamic simulation software, such as IES-VE, is the level of complexity that a user can go into with their modelling. If we were just to look at occupancy profiles, these are formed by creating a daily profile, which is then used to formulate a weekly profile and builds up to an annual profile. To apply this to a practical example, with a Main Hall at a Primary School, it is likely that this could be used for 30min assemblies with the whole school, a single class doing PE, lunch time dining, after school activities, special school events or even be used by the local community during some evenings or weekend.

Another common conundrum is relating to modelling a room based upon a maximum occupancy or a peak period of internal gain that may only occur either occasionally during the year or for short periods of time. Instead of modelling the room based upon this worst case scenario, which could lead to over engineering of the scheme, with the use of a dynamic simulation, the standard room use and the occasional peak room use, can both be incorporated into one model. The peak load could then be applied to the room once a week, once a month or even for a couple consecutive days during one week in July.

Natural Ventilation design

A key concern that is raised by clients in relation to natural ventilation systems is ‘How will the systems provide fresh air when there is no external wind?’ or ‘What happens when during a hot summer’s day?’. By carrying out a dynamic simulation, using a specific weather file, this will take into account the periods of hot, sunny days that would normally be experienced in the summer months for that particular location, or the variable external wind speeds and directions that will occur throughout the year.

As the UK Design criteria generally stipulates a capability of providing 8 or 10 l/s per person, even within a dynamic simulation, it is a common misunderstanding that natural ventilation can be just applied to a room as a set ventilation rate of 8 or 10 l/s per person, on the assumption that a natural ventilation system or opening will provide this requirement. Although with a steady state calculation, the sizing of the natural ventilation systems is generally based upon specific conditions, such as the external and internal temperature difference, a conservative wind speed value and a specific wind direction. The problem however is that these are all variable figures and will differ during the year, with the wind varying in speed between 0 to 90 mph and the direct having 365°.

By carrying out a dynamic simulation, the natural ventilation systems and openings can be sized against specific weather data for the required project site. This allows rooms on different elevations of the building to be modelled against the variable wind direction, whereby one elevation may face the prevailing wind most of the year, the elevation on the opposite side of the building may have very little direct wind. Therefore, for rooms located on opposite sides of a building, although they may be completely identical, they will experience very different levels of ventilation.

Post-Occupancy evaluation

The use of thermal dynamic modelling can also open the way to carrying out post-occupancy simulations and evaluations. Throughout the UK, the UK Meteorological Office collects data from more than 3600 locations around the UK and the data from a desired period of time can be collated into a weather file suitable for running a dynamic simulation. The design stage model that was then simulated against a predicted weather file, prior to the construction of the building, can then be simulated against the real life weather file data.

As with many Building Management systems, Monodraught’s ventilation systems are provided complete with inbuilt data logging facilities as standard and part of the annual service that is provided includes analysing the system performance within its installed environment during the desired period. With the large amount of data that is recorded, analysis can be carried out comparing the performance of the design simulation against the real life recorded building performance data. Although it can be expected to see minor variations, however should there be a large variation between the compared results, further examinations can be undertaken to identify the cause of this difference.Some of these differences could be down to how the particular room in used in reality compared to how it was designed, differences in the type of construction materials/build up, or even a problem with the operation of the heating or ventilation systems.

The Future of Dynamic Simulation

With the use of new dynamic simulation software developments, it will be possible to then integrate the real life room or building data, that has been recorded via the particular control/BMS system, into the design stage modelled building to allow an even more detailed analysis of exactly what occurring on a much larger scale. As opposed to analysing a building or system on a room by room basis from columns of data or graphical outputs, the data could be examined from the building as a whole. This would allow designers and engineers to identify periods where the building or systems may not be performing as it has been designed to do, then examine the causes of this anomaly and if the problem in one room is causing a knock-on effect in an adjacent room.

Aside from identifying and assisting in resolving problems, the ability to integrate real life performance data into a dynamic simulation could allow designers and manufactures develop and improve their systems. For ventilation systems, this data could be used to identify periods where the system may be over performing, so dynamic simulation tests could be carried out to analyse the effects of providing less ventilation to a room or adjustments to the internal room set points.

To complete the CIBSE module visit the CIBSE journal website, read the full module and answer the questions to gain CPD credits.

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